Are horseshoe crabs…crabs?

Summary

But Why: A Podcast for Curious Kids Episode Summary: Are Horseshoe Crabs… Crabs? Release Date: May 30, 2025

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Introduction

In this captivating episode of But Why: A Podcast for Curious Kids, host Jane Lindholm takes listeners on an exciting field trip to Cape Cod, Massachusetts, alongside Sarah Grady, a Senior Coastal Ecologist with Mass Audubon. The primary focus of their exploration is the enigmatic horseshoe crab—a creature that has roamed the Earth for over 400 million years. Throughout the episode, Jane and Sarah delve deep into understanding why horseshoe crabs are named as such, their unique biology, their role in the ecosystem, and their interactions with humans.

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Field Trip to Cape Cod: Observing Horseshoe Crabs

[00:00 - 03:31] Jane Lindholm sets the scene by describing Cape Cod's picturesque beaches, likening the area's shape to a bent arm showcasing bicep muscles. As they arrive at the beach, Jane narrates their first encounter with what initially appear to be large rocks but reveal themselves to be live horseshoe crabs adorned with barnacles and shells.

"We're heading out to a beach not far from where your elbow would be. Just above your elbow, but not near your wrist. It's a little too early to swim. The water is really cold... But it's a beautiful sunny day, and when we arrive it's almost high tide and we can see what look from afar kind of like big rocks in the water." [02:15]

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Questions from Kids

Several curious children have submitted questions for this episode, primarily focusing on the nomenclature and nature of horseshoe crabs:

• Benicio (9, Silver Spring, Maryland): "Why are horseshoe crabs called horseshoe crabs?" [03:31]
• Amir (9, Limerick, Ireland): "Why are horseshoe crabs called horseshoe crabs?" [03:31]
• Ava Rose (8, Vancouver, Canada): "Why are horseshoe crabs called horseshoe crabs if they don't look like crabs?" [03:31]
• Additional questions regarding their appearance, behavior, and biology are also presented throughout the episode.

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Interview with Dr. Sarah Grady

Sarah's Introduction and Role

Sarah Grady introduces herself as a Senior Coastal Ecologist working with Mass Audubon, specializing in the study of coastal ecosystems and horseshoe crabs.

"We are at a little tucked away beach." [04:15]

Biology of Horseshoe Crabs

Sarah provides an in-depth look at the anatomy of horseshoe crabs:

"The crab has three parts to its body. So it has this front rounded part, it's called the prosoma. And then this part here, this sort of, I don't know, trapezoid shaped part is called the Apistosoma. And it has these little spines on the side. And then this long pointy part, the tail, is called the Telson." [07:11 - 07:40]

Jane further explores the number of claws and their functions:

"They are 10 claws. Their most sensitive part are these flaps here which are their gills." [09:11 - 10:20]

Why They're Called Horseshoe Crabs

Addressing the children's primary question, Sarah explains the origin of their name:

"Well, if you look at the front shape of the horseshoe crab, it looks like a horseshoe. It's a U shape and that's. That's it. Yeah." [07:46 - 08:04]

She further clarifies why they're termed "crabs" despite not being true crabs:

"Most people, I think when they see something that has claws and is walking around in the ocean will think that it is a crab of some sort. Most crabs that you see are going to walk sideways. Horseshoe crabs are walking forward. They are more closely related to spiders and scorpions." [10:20 - 11:05]

Jane reflects humorously on this misnaming:

"Let me just pause here for a second. Did you know that horseshoe crabs aren't really crabs? I didn't. They're more closely related to spiders and scorpions, as she said. So we've been calling them the wrong thing all along. At least we got the horseshoe part right." [11:05 - 11:32]

Survival Through Millions of Years

The discussion shifts to the remarkable longevity of horseshoe crabs:

"They've been around for about 450 million years. Yeah. So they are one of the oldest living creatures walking around here." [11:45 - 11:57]

Sarah attributes their survival to their robust body shape and minimal natural predators:

"They survived by being tough and I think really finding the right body shape. Crabs tend to do really well in general... They really don't have a lot of natural predators when they're in the water, and so they can just keep going." [12:04 - 12:35]

Reproduction and Lifespan

Exploring their mating rituals, Sarah describes how horseshoe crabs lay eggs:

"The female will dig a nest and actually that female is doing that right now. So she's digging in the sand, she's going to lay her eggs and then they're going to move forward a little bit and then the male will fertilize those eggs and in about two weeks those eggs will hatch out." [05:30 - 05:48]

They discuss the impressive number of eggs a single female can lay:

"I read that a female Horseshoe Crab can lay 80 to 100,000 eggs a year." [05:48 - 05:54]

Regarding lifespan:

"So they take about 12 years between the time that they hatch out to the time that they come back to the beach to lay their eggs. And so that means that anything that's done to protect them when they're laying their eggs, we're not going to see if the protections worked really well until those crabs grow up and come back." [28:30 - 29:23]

Sarah estimates their total lifespan to be around 25 to 30 years, noting their molting process slows as they mature:

"They can live up to about 25, 30 years, we think." [30:11 - 30:33]

Interactions with Humans: Consumption and Harvesting

Addressing human interactions, Sarah explains that while humans rarely consume horseshoe crabs directly, they are harvested for two main purposes:

1. Bait for Whelk Fishery:

   • "One of the reasons that horseshoe crabs are harvested is for bait, and that's for the whelk fishery. So a whelk is a giant snail and they...they love the smell of horseshoe crabs, especially horseshoe crab eggs." [24:25 - 24:42]

2. Blood Harvesting:

   • "Horseshoe crab blood has a very special property. They have a really sensitive immune system... a small amount of it can be used to test whether that is sterile." [24:43 - 26:50]

Sarah highlights ongoing efforts to find synthetic alternatives to horseshoe crab blood to prevent overharvesting:

"There's an effort to find a synthetic alternative so that horseshoe crabs don't have to be harvested at all for that purpose." [26:50 - 26:55]

She further emphasizes the need for humane treatment during harvesting and the development of synthetic compounds:

"Researchers are trying to figure out how to make sure the crabs are treated fairly...find ways to make a chemical that does what the horseshoe crab blood does so they don't need to endanger the crabs at all." [27:22 - 27:57]

Unique Features: Blood Color and Properties

A fascinating aspect of horseshoe crabs is their blue blood, explained by Sarah:

"Their blood is blue. Yeah, it's a sort of a sky blue color...because the molecule that's used to carry oxygen in horseshoe crab blood is based on copper. It's called hemocyanin." [27:30 - 28:11]

She contrasts it with human blood, which is red due to iron-based hemoglobin:

"Our blood is red. And that molecule in our blood has iron in the middle. So, Jane, what color does iron turn when it's exposed to oxygen? Rusty red." [28:11 - 28:16]

Further analogies help visualize the color difference:

"So that's why their blood is blue... or the Statue of Liberty." [28:16 - 28:32]

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Insights and Conclusions

Sarah Grady and Jane Lindholm conclude the episode by emphasizing the ecological significance of horseshoe crabs. Their eggs are a vital food source for migratory shorebirds, such as the Red Knot, which relies on these eggs to fuel their long journeys from the Arctic to South America.

"Not all of the eggs end up buried under the sand... some of them can provide lots of food for shorebirds, like the Red knot." [18:32 - 19:08]

They also discuss the current status of horseshoe crab populations, which vary by region. Conservation efforts, including prohibiting harvesting during spawning, are crucial for the species' recovery and sustainability.

Jane offers practical advice for young listeners:

"If you're interested in looking closer at a horseshoe crab, it's a good idea to pick it up and get to know it a little bit. Make sure you put it back in the water... hold it like a bowl of cereal in your hand." [15:36 - 16:01]

She encourages participation in local conservation projects, urging children and their families to engage with wildlife organizations to support horseshoe crab preservation.

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Notable Quotes

• Jane Lindholm: "Did you know that horseshoe crabs aren't really crabs? I didn't. They're more closely related to spiders and scorpions, as she said. So we've been calling them the wrong thing all along. At least we got the horseshoe part right." [11:05 - 11:32]

• Sarah Grady: "Their blood is blue...because the molecule that's used to carry oxygen in horseshoe crab blood is based on copper. It's called hemocyanin." [27:30 - 28:11]

• Sarah Grady: "Horseshoe crab blood has a very special property. They have a really sensitive immune system... a small amount of it can be used to test whether that is sterile." [24:43 - 26:50]

• Jane Lindholm: "So they're not doing it to be mean." [15:26]

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Conclusion

This episode of But Why: A Podcast for Curious Kids offers an engaging and informative exploration into the world of horseshoe crabs. Through vivid descriptions, expert insights from Dr. Sarah Grady, and direct engagement with questions from young listeners, Jane Lindholm effectively demystifies these ancient creatures. The episode not only educates but also inspires young minds to appreciate and participate in wildlife conservation efforts, highlighting the interconnectedness of all species within our ecosystems.

Transcript

[Jane Lindholm]

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[Sarah Grady]

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[Jane Lindholm]

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[Curious Kid]

I'm Benicio, I live in Silver Spring, Maryland. I'm nine years old. Why are horseshoe crabs called horseshoe? My name is Amir, I'm nine years old and I live in Limerick, Ireland. My question is why are horseshoe crabs called horseshoe crabs? Hi, my name is Ava rose and I'm 8 years old and live in Vancouver, Canada. Why are horseshoe crabs called horseshoe crabs if they don't look like crabs?

[Jane Lindholm]

You've sent us a lot of horseshoe crab questions. So it's a good thing we've met up with a scientist who studies these incredible arthropods because she can answer them all.

[Sarah Grady]

I'm Sarah Grady, I'm the senior coastal ecologist for Mass Audubon. We are at a little tucked away beach.

[Jane Lindholm]

I see what looks like a little hump of something heading towards the shore. Is that a horseshoe crab?

[Sarah Grady]

That is a horseshoe crab. They're coming into the shore as the tide is coming in so that they can lay their eggs.

[Jane Lindholm]

And that's what we're here to see. So that's very exciting to me. Is it exciting to you as a horseshoe crab expert?

[Sarah Grady]

It's always very exciting. What's most exciting is when you see a pair of them because they form these pairs of a female and a male and they walk around together.

[Jane Lindholm]

Wait, there's three here?

[Sarah Grady]

Yeah, yeah. So they, this is a, a pair of crabs. So there's a larger crab in the front, that's the female, and then there's a smaller crab in the back that is a male. And then there's a second male that has been attracted to the action that's going on and is trying to maybe.

[Jane Lindholm]

Get involved just to be clinical about this. The horseshoe crabs are mating and this is how they make more horseshoe crabs.

[Sarah Grady]

So what happens is the female will dig a nest and actually that female is doing that right now. So she's digging in the sand, she's going to lay her eggs and then they're going to move forward a little bit and then the male will fertilize those eggs and in about two weeks those eggs will hatch out.

[Jane Lindholm]

I read that a female Horseshoe Crab can lay 80 to 100,000 eggs a year.

[Sarah Grady]

Yep, yep. They are pretty much a walking box of eggs, actually. And the eggs are a really beautiful greenish blue color. They're very tiny, about 2 millimeters, so about the size maybe the end of a pencil lead. And. Yeah, they lay them in little clusters in the sand.

[Jane Lindholm]

So the female is moving forward right now, apparently dragging both of these males. What, so she, she's already laid some eggs?

[Sarah Grady]

She, she may have laid some eggs already. Yeah. Yep.

[Jane Lindholm]

There looks like there's another male coming along for the chase now.

[Sarah Grady]

Yeah. Yes. There are usually a lot more males than females. So if there's a female, she almost always has a male attached to her. And then there will be other males that, that are coming along for the ride.

[Jane Lindholm]

I see another male over there. It's like they're all racing to get to this one female.

[Sarah Grady]

Yes. Yeah. And when we see a pair that is not buried in the sand, we'll be, we can pick them up and I can show you there might be.

[Jane Lindholm]

A pair over there. We waited in the shallows to where we saw two crabs scrap scuttling towards the edge of the water. Sarah picked one up so we could get a better look at this unique creature.

[Sarah Grady]

The crab has three parts to its body. So it has this front rounded part, it's called the prosoma. And then this part here, this sort of, I don't know, trapezoid shaped part is called the Apistosoma. And it has these little spines on the side. And then this long pointy part, the tail, is called the Telson. So they have these three body parts.

[Jane Lindholm]

One of the main questions that we've gotten from kids is why are horseshoe crabs called horseshoe crabs?

[Curious Kid]

My name is Ivy, I'm six years old. I live in Gilbert, Arizona. And why does horseshoe crab start with a horseshoe? My name is Jack. I'm seven years old and I live in Belfond, Pennsylvania. Where do horseshoe crabs get their name?

[Sarah Grady]

Well, if you look at the front shape of the horseshoe crab, it looks like a horseshoe. It's a U shape and that's. That's it. Yeah.

[Jane Lindholm]

And the horseshoe part, that, that big armored part really helps protect a lot of their organs, right?

[Sarah Grady]

Yeah. So they're really well protected, both on the back of their shell, the formal name being a carapace for the crab. They're also really well protected underneath from, for the most part, if you look on the underside, they have all of these different claws here, and they're different on a male in particular. So we Know that this is a male horseshoe crab because his front claws look like boxing gloves. So those are called boxer claws. And then the rest of the look like scissors. If this was a female, all of the claws would look like scissors. They also have these special rear claws that actually are more like ski poles.

[Jane Lindholm]

Ski. Yeah, they have like five little appendages that come out of it. These are wild looking.

[Sarah Grady]

Yeah. So this, you know, just like a hiking pole or a ski pole, helps the crab push themselves forward as they walk across the mud. And then these two tiny claws here are called chelicerae. And that's how they push their food into their mouth. So this is the horseshoe crab's mouth, this spiky hole here. So they use these claws to kind of push stuff down in there so that they can eat it.

[Jane Lindholm]

How many claws does one crab need?

[Sarah Grady]

I know, right, so let's see, let's count them. So we've got our two special front claws on the male. Then we've got another 1, 2, 3 on each side. So we're up to 8. And then we've got these pusher claws. So that brings it up to 10, right. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. And these little tiny ones don't really count. Those are. So, yeah, 10 claws. Their most sensitive part are these flaps here which are their gills.

[Jane Lindholm]

So the reason they're called horseshoe crabs is somewhat self explanatory. The main carapace looks like a horseshoe. But why are they called crabs? They're not really crabs, are they?

[Sarah Grady]

Right. Well most people, I think when they see something that has claws and is walking around in the ocean will think that it is a crab of some sort. Most crabs that you see are going to walk sideways. Horseshoe crabs are walking forward. They are more closely related to spiders and scorpions. And in fact, these little claws, if you were to look up close on a spider, you would also see those on a spider.

[Jane Lindholm]

So they're sort of named crabs, not by mistake, but by humans just making assumptions.

[Sarah Grady]

Right, right. Just like, you know, there are other marine creatures that are misnamed, like starfish and jellyfish, neither of which are fish. Right.

[Jane Lindholm]

Let me just pause here for a second. Did you know that horseshoe crabs aren't really crabs? I didn't. They're more closely related to spiders and scorpions, as she said. So we've been calling them the wrong thing all along. At least we got the horseshoe part right.

[Curious Kid]

My name is Emilio. I'm six years old. I live in Columbus, Ohio. How did horseshoe crabs live before dinosaurs and how are they still alive today?

[Jane Lindholm]

How long have horseshoe crabs been alive?

[Sarah Grady]

They've been around for about 450 million years. Yeah. So they are one of the oldest living creatures walking around here. Yeah.

[Jane Lindholm]

How did they survive that long? How did they survive when dinosaurs didn't? And, I mean, what's the secret of the horseshoe crab?

[Sarah Grady]

There have been a lot of extinctions between the time that the horseshoe crab evolved and now. And they survived by being tough and I think really finding the right body shape. Crabs tend to do really well in general, even the later crabs in evolution. So it's a good body to have. They really don't have a lot of natural predators when they're in the water, and so they can just keep going.

[Jane Lindholm]

Does this one need to stay wet?

[Sarah Grady]

That's a great question. So part of coming into spawn is that they often will end up out of the water. And they have these special gills called book gills. And they're almost like the pages of a book. So there's these flaps on top, but then within each of the tougher flaps, there's pages almost. And so those book gills will allow the crab to breathe as long as those can stay wet so the. They can pull oxygen out of the air through the water that's on their book gills.

[Jane Lindholm]

So their book gills will stay wet for a little while at least. Even if they're out two days.

[Sarah Grady]

They. Yeah.

[Jane Lindholm]

Oh, that's a long time.

[Sarah Grady]

Yeah, they. They're okay for about two days or so. Of course, when they're on the beach, they are more vulnerable. A gull might.

[Jane Lindholm]

Sarah, watch out. There's something coming for you. Right behind you. Look out behind you.

[Sarah Grady]

Oh, look what we have right here. Yes. Another. Another crab. This one looks a little bigger. So I'm gonna see. Maybe it might be a female. Nope, another male.

[Jane Lindholm]

One of the other things I notice about this one that you're holding is that there are some things attached to it, other shells, other creatures attach themselves to horseshoe crabs and get a ride.

[Sarah Grady]

Yeah. So they have these slipper shells that are on the back of the crab. Sometimes they'll have barnacles. This one has some algae on it. And horseshoe crabs actually have a flatworm that lives on mostly their gills. It's called a limulus leech. Limulus is the genus scientific name for horseshoe crabs, and it only lives on horseshoe crabs. And it's actually a flatworm. It's not a leech.

[Curious Kid]

My name is Louisa. I'm four years old. I want to know if horseshoe crabs pinch. I am from Wayne, Pennsylvania.

[Jane Lindholm]

Do horseshoe crabs pinch?

[Sarah Grady]

They don't. Nope. I'll show you right now that you can put your finger right in the claw.

[Jane Lindholm]

I'm gonna do it too.

[Sarah Grady]

Yeah, you can pick a claw, any claw, and stick your finger in there if you can get it.

[Jane Lindholm]

It won't even grab me. It's trying to grab the microphone. It's like, what are you doing?

[Sarah Grady]

Yeah, they're, they're. They're very friendly, actually. You wouldn't think so, but. But yeah. One of my favorite things to do, actually, is if you kind of put your hand right in the middle of them, they kind of will give you a hug with their claws. Oh, I think it just poked your microphone with the. With its tail. So one thing I will say if you are picking up a horseshoe crab is they do like to fold in half. So don't put your fingers between the two halves of the shell, because that is the one way they can pinch you.

[Jane Lindholm]

But they're not doing it to be mean.

[Sarah Grady]

No, they're just folding in half. And you happened to have your hand in the wrong place.

[Jane Lindholm]

Should we be picking up horseshoe crabs? Is it okay to be picking them up and making them give you a hug?

[Sarah Grady]

That's a really good question. I think if you're interested in looking closer at a horseshoe crab, it's a good idea to pick it up and get to know it a little bit. Make sure you put it back in the water. Don't ever pick a horseshoe crab up by its tail. You want to pick it up by the front by that rounded horseshoe part, and you can hold it just like a bowl of cereal in your hand. So that's going to keep it.

[Jane Lindholm]

That's sort of upside down, then upside down.

[Sarah Grady]

And they like to. They like to fold up in half. So you can just hold the crab like that. And you don't want to disturb any crabs that are actively mating.

[Curious Kid]

Hi, I'm Gray. I'm five years old. I live in Larkspur, California. Why do horseshoe crabs have the pointy thing on the bottom?

[Jane Lindholm]

Tell us a little bit about this very long tail on the horseshoe crab.

[Sarah Grady]

If you imagine that the crab has ended up on its back and it's stuck on the sand, or maybe in a little bit of water, it needs to kind of pole vault itself back over. So that's what that tail is for. It kind of will twist and plant the end of that in the sand or the mud and flip itself over.

[Jane Lindholm]

But it's so long. And you're not supposed to pick them up by it. Do they get ripped off or lost?

[Sarah Grady]

They do sometimes. Yeah. Yeah. Sometimes it will get caught and stuff, and then you'll just see they have a little stub or something like that.

[Jane Lindholm]

We know that horseshoe crabs don't pinch, but can they sting you with that tail?

[Sarah Grady]

Nope, nope. They are not gonna sting you. They're not gonna try to hurt you with that tail. They don't have any sort of venom or anything like that.

[Curious Kid]

My name is Quinn. I'm five years old. I'm from Lockett Center, New York. What do horseshoe crabs eat?

[Jane Lindholm]

What do horseshoe crabs eat?

[Sarah Grady]

They eat a variety of little creatures that live in the mud.

[Jane Lindholm]

So they're carnivores?

[Sarah Grady]

They are, yes. Yep. They eat little worms. They will eat small shrimp, like animals called amphipods that live in the mud, and they also will eat small shellfish. And in fact, that's kind of what got horseshoe crabs in trouble for a while with people, because everybody thought that the horseshoe crabs, especially on Cape Cod, where we are, were eating all of their clams, and they got really upset with the horseshoe crabs, and you could actually kill the horseshoe crabs and you would take their tail to the town hall and they would pay you a nickel.

[Jane Lindholm]

Wow.

[Sarah Grady]

Yeah. Yep. So they do eat some little clams and things like that, but it's not a huge part of their diet.

[Jane Lindholm]

So now we know they're not actually eating all the clams that the humans would like to be eating.

[Sarah Grady]

Yes, that's right. Yeah.

[Jane Lindholm]

What eats horseshoe crabs?

[Sarah Grady]

Not a lot, but something that's really important that they do is feed migratory shorebirds. So when horseshoe crabs lay their eggs, not all of the eggs end up buried under the sand where they're going to turn into new horseshoe crabs. Instead, some of them end up closer to the surface, and there they can provide lots of food for shorebirds, like the Red knot, which is a bird that migrates all the way from Argentina, from the Tierra del Fuego, way, way south, all the way up to the Arctic.

[Jane Lindholm]

Shall we look at some more horseshoe crabs and put this guy back and let him get on with the process of finding someone to mate with?

[Sarah Grady]

Sure. Sounds good.

[Jane Lindholm]

Coming up. We know what horseshoe crabs eat and what eats them. But do we eat horseshoe crabs? And if not, why are people catching them? But first, a message for the adults who are listening. Support for our program comes from Oak Meadow. Providing secular student centered homeschooling curriculum and a teacher supported distance learning school for K through 12. Oak Meadow has encouraged kids to follow their curiosity and uncover the answers to but why? For 45 years. To learn more, visit oakmeadow.com this is but why? A podcast for curious kids. I'm Jane Lindholm. We're on a field trip to Cape Cod with scientist Sarah Grady, a coastal ecologist who works with Mass Audubon. Sarah is teaching us all about horseshoe crabs and answering the many questions you sent in for us to ask her. In the spring, you can sometimes find these arthropods who, remember, aren't really crabs at all. Close to shore, they usually live a little deeper out, spending most of their lives on the seafloor, away from human eyes. But in May and June, they gather in big groups at the edge of the water to mate and lay eggs. You can see them all up and down the eastern coast of the United States, and the best time to spot them mating is high tide, when there's a flat full moon or a new moon. On the day we were visiting Cape Cod to see them, there was a citizen science project going on where volunteers were counting as many crabs as they could to try to keep tabs on how these ancient arthropods are doing. As we were talking, we spotted a whole bunch of horseshoe crabs in the shallows around our feet. They actually seemed pretty curious about us. Here's the whole train of them here.

[Sarah Grady]

I just want to show you. We'll put these right back, but I can pick them all up at once.

[Jane Lindholm]

She's so much bigger than the males.

[Sarah Grady]

Very big, yes.

[Jane Lindholm]

How big do they get?

[Sarah Grady]

Well, the biggest ones are down in Delaware Bay. They get bigger as you go south toward that area, and then they get smaller again as you go down toward the Gulf of Mexico. They can be over a foot across, plus. Yeah.

[Jane Lindholm]

So this one's big, but she's not big.

[Sarah Grady]

She's pretty big. She's pretty big for horseshoe crabs in Massachusetts. I'm going to put these three back. And as you notice, the tide has come in since we've been talking, too.

[Jane Lindholm]

Yeah.

[Sarah Grady]

So that's what. That's what they're following the incoming tide.

[Jane Lindholm]

So why do they want to lay their eggs up here? Sometimes it looks like not totally out of the water, but. And sometimes out of the water here up at high tide.

[Sarah Grady]

Well, it's a place where once those eggs are buried in the sand, they won't get washed away as often. Right. So if they were to make their little nest under the water every time there was a wave, it would be kind of messing that nest up this way. It's up at the edge of the Water. And the water will get there, but not as often.

[Jane Lindholm]

And.

[Sarah Grady]

And they also, in places especially like, again, down in the mid Atlantic, they really prefer to lay their eggs around the highest high tides. So those would be the tides that occur when we have a new or full moon. It's called a spring tide. And so that way that nest isn't going to really have water on it for two weeks, which is just about the amount of time it takes for the eggs to hatch.

[Jane Lindholm]

How tiny are the babies when they hatch? What do they look like?

[Sarah Grady]

They are called trilobite larvae, and they look like a mini horseshoe crab. They're only, again, I mean, they're just a little bit bigger than the egg they were in. So they're only about 2 or 3 millimeters across. And they look exactly like a tiny horseshoe crab, except they don't have a tail. And the reason they don't have a tail is it's usually not a good idea to have something sharp and pointy inside your egg.

[Jane Lindholm]

True. I hadn't thought about that.

[Sarah Grady]

Yeah, yeah, so grow that part once you're, you know, safe outside the egg.

[Curious Kid]

Hi, my name is Jude and I'm five years old and I live in Vancouver, Canada. Humans eat horseshoe crabs.

[Jane Lindholm]

Do humans ever eat horseshoe crabs?

[Sarah Grady]

Not really. There's a very small market for eating the eggs. Very, very small. Just like, you know, if you've ever ordered sushi with fish eggs on the outside or something like that, it's similar to that, but that's mostly the Asian horseshoe crabs. It's not like we're exporting American horseshoe crab eggs to Asia.

[Jane Lindholm]

However, humans do capture horseshoe crabs and kill them and use them. Why are horseshoe crabs important to humans?

[Sarah Grady]

So right now, there are two reasons that horseshoe crabs are harvested. One of them is for bait, and that's for the whelk fishery. So a whelk is a giant snail and they.

[Jane Lindholm]

Which people do eat.

[Sarah Grady]

Which they do eat. Yes. Yep. And there's a market for that. And so they love the smell of horseshoe crabs, especially horseshoe crab eggs, so females will catch more whelks. And so there's an industry for collecting horseshoe crabs for bait. Fortunately, you're no longer allowed to harvest horseshoe crabs while they are spawning, which is going to help the population grow. The other reason that horseshoe crabs are harvested is for their blood. So horseshoe crab blood has a very special property. They have a really sensitive immune system. And so anytime any sort of bacteria gets into a horseshoe crab, if you can imagine the horseshoe crab getting a cut through that big shell it's able to immediately form a clot around that wound and seal it up. And people discovered that you could actually take that blood and get that particular chemical from their immune system and use that to detect whether something had bacteria in it. So if you had a company and your job was making things like saline that they, you know, you put in an IV or maybe a vaccine before you had to test to see if it had bacteria in it or was contaminated by injecting it into a rabbit and waiting to see if the rabbit got a fever. Now, this compound from horseshoe crab blood, a small amount of it, can be used to test whether that is sterile. So that's been really helpful. And there's an effort to find a synthetic alternative so that horseshoe crabs don't have to be harvested at all for that purpose.

[Jane Lindholm]

When you say synthetic, you mean a human made version that doesn't require killing any horseshoe crabs to get it.

[Sarah Grady]

Exactly.

[Jane Lindholm]

To take the horseshoe crab blood, crabs are captured and some of their blood is drained out. The crabs are then put back in the water. But not all of them survive that whole procedure. So researchers are trying to figure out how to make sure the crabs are treated fairly. And as Sarah mentioned, they're trying to find ways to make a chemical that does what the horseshoe blood does so they don't need to endanger the crabs at all.

[Curious Kid]

My name is Hazel. I'm eight years old. I live in Gilbert, Arizona. My question is, what color is horseshoe crab's blood?

[Sarah Grady]

Their blood is blue. Yeah, it's a sort of a sky blue color, I suppose, a lighter blue. And that is because the molecule that's used to carry oxygen in horseshoe crab blood is based on copper. It's called hemocyanin. And it's kind of like the hemoglobin that's in our blood. So we have red blood cells. Our blood is red. And that molecule in our blood has iron in the middle. So, Jane, what color does iron turn when it's exposed to oxygen?

[Jane Lindholm]

Rusty red.

[Sarah Grady]

Yeah. And what color does copper turn when it gets too much oxygen?

[Jane Lindholm]

Kind of a blue, which is confusing because if you see a penny, you think it's a copper color, which is like an orangey metallic. But if you've ever seen a copper roof, after a long time, it's kind of a sea blue.

[Sarah Grady]

Yeah. Or the Statue of Liberty.

[Jane Lindholm]

Oh, yeah, I forgot about that.

[Sarah Grady]

Yeah. Yep. So that's why their blood is blue.

[Jane Lindholm]

So are horseshoe crab populations in trouble?

[Sarah Grady]

It depends on the place. In a lot of places they have been over harvested And a lot of people have worked hard to try to protect them.

[Jane Lindholm]

So what's the status now?

[Sarah Grady]

They are increasing in some places, stable in others. And then there are other places where they have declined. So it's kind of all over the place. Horseshoe crabs take about 12 years between the time that they hatch out to the time that they come back to the beach to lay their eggs. And so that means that anything that's done to protect them when they're laying their eggs, we're not going to see if the protections worked really well until those crabs grow up and come back.

[Curious Kid]

My name is Sophie. I'm five years old. I live in Bellefontein, Pennsylvania. How long can horseshoe crabs live?

[Jane Lindholm]

So 12 years until they are adults, essentially, until they're mature and can make new horseshoe crab babies. How long do they live in total?

[Sarah Grady]

It's hard to know because unlike something like a fish or like even a clam or a tree, you can't cut it and count rings or count layers on various bones. They don't have bones like that. So we have to kind of go by size and also to some degree, by the things that are growing on the shell. Can also be used to kind of get somewhat of an age.

[Jane Lindholm]

You mean, like, you'd be like, look, dude, you have that barnacle. That barnacle's at least seven years old, which means you must be even older.

[Sarah Grady]

That's right. You've had that jewelry for a long time.

[Jane Lindholm]

But I thought horseshoe crabs molted and got rid of their shells.

[Sarah Grady]

The molting slows down once they become mature, so they can live up to about 25, 30 years, we think. Yeah. Yeah.

[Jane Lindholm]

And how often do they molt in a lifetime?

[Sarah Grady]

Oh, it's about 15 times.

[Jane Lindholm]

So they get a new shell 15 or 16 times. But most of that is when they're really young.

[Sarah Grady]

Yeah, they molt six times in their first year. Yep. And they grow from being, you know, about the size of a kind of a dull pencil lead all the way up to being bigger than a quarter.

[Jane Lindholm]

Wow. In one year. That's a lot. I mean, it still sounds pretty small, but if you're a crab, that's a lot of growth.

[Sarah Grady]

Yeah, yeah. Yep.

[Jane Lindholm]

There's like a ton of them over there.

[Sarah Grady]

All right, let's keep moving. Oh, here's what. Here's a. Here's a pair that's actually buried in.

[Jane Lindholm]

Oh, yeah. Wow.

[Sarah Grady]

So she came in, she's dug this nest, she's laying the eggs, and then they'll probably leave in about an hour.

[Jane Lindholm]

So they're out here on the very edge where we can see them right now and there are so many of them. It's so cool because I never see horseshoe crabs and I'm amazed. But where are they normally?

[Sarah Grady]

Well, when it's not time for spawning, they stay in the deeper water and they're just out there eating because they're cold blooded. They do slow down in the wintertime so they'll kind of dig into the mud and stay there. And that's actually when they get a lot of the barnacles and things on them because they pretty much turn into a rock for a while.

[Jane Lindholm]

We figured we'd better let the crabs get on with their mating and spawning. But I had to get Sarah to do one more thing while I had a microphone in her face. Sarah has written a whole song about horseshoe crabs. It's a specific type of song called a sea shanty. And I sprung it on her as a surprise that I knew about the song and wanted to see if she remembered enough to still be able to sing a verse.

[Sarah Grady]

A little bird called the red knot can fly so far, yes, quite a lot. From South America to the Arctic. On their migration while they fly, they stop to rest. They think that horseshoe crab eggs are best for getting fat and bulking up and reaching satiation. Feeding this little bird is quite important. I'm sure you've heard how these two animals are strongly linked together. Their blood is blue. That's because of copper. It clots up as an infection stopper.

[Jane Lindholm]

I was liking that verse.

[Sarah Grady]

Oh yeah, I know I need copper. Oh yeah. I can't, I can't remember.

[Jane Lindholm]

That was great. Congratulations. Yay.

[Sarah Grady]

Yay. Yay.

[Jane Lindholm]

Claps I've had that song in my head ever since we got Sarah to sing it. Thanks to Dr. Sarah Grady, Senior Coastal Ecologist with Mass Audubon, for sharing her deep knowledge of horseshoe crabs with us. We're going to put some videos on our YouTube channel and social media channels if you want to see what these crabs look like. If you're lucky enough to see a horseshoe crab, observe them very carefully. They are wild animals and they deserve to be treated with respect. They're not afraid of people and they may come right up to you. Remember, they don't pinch and that tail doesn't have any venom or any way to sting you. If you want to pick one up though, be very careful. Hold it like a bowl of cereal and don't put your fingers in between the two segments of the body because when they fold you can get your fingers stuck where their body parts hinge and don't keep them out of the water for too long. Put them back in when you're done observing and make sure they're not on their backs. And if you live near the coast, there may be a survey happening near you. Have your adults help you check with your state and local wildlife organizations to see if you can get involved in horseshoe crab conservation efforts. That's it for this episode. If you have a question about anything, have an adult record you asking it. It's easy to do on a smartphone using an app like Voice Memos. If you are too shy to talk or can't use your voice, you can always have an adult email us a written question and we can use that too. Be sure to tell us your first name, where you live and how old you are. Then have your adult email the file to questionsutwhykids.org but why is produced by Melody Beaudet, Sarah Bake and me, Jane Lindholm at Vermont Public and distributed by PRX. Our video producer is Joey Palumbo. Check out our YouTube series but why Bites. We put out a short bite sized video episode every other Friday and we're about to put one out about horseshoe crabs. Our theme music is by Luke Reynolds if you like. But why? Please like and comment wherever you listen and and share with your friends. It really helps other people discover us. And if you really like boatwei, you can get a Boatwy T shirt, a boatwei water bottle and other cool swag at our website. But whykids.org we'll be back in two weeks with an all new episode. Until then, stay cur.

[Sarah Grady]

From PRX.